Unfortunately, the most inexpensive one that I've seen is the Kanax adapter, which is about the same as an eSATA external drive case.
Ironically, some older PCs and laptops have a couple of eSATA ports that work fine with the eSATA case I have. In fact, that's how I was able to upgrade the Crucial SSD firmware on a older PC ThinkPad laptop.
It's a real shame that Thunderbolt is relatively so expensive compared to USB 3.x; even a Thunderbolt cable is what, $40 about; and Thunderbolt cases and drives are all more expensive than a USB case or drive.
It makes it unfortunately difficult to take advantage of the new trim enable command in 10.10.4 and 10.11 beta. An external drive is so much easier to work with, rather than having to install a SSD internally on a mac.